The present invention relates to a method and device that produces a high power, broadband optical output, and more particularly to a device for producing a broadband polarized optical output with very low relative intensity noise (RIN).
It is necessary and desirable in certain applications and circumstances to have the capability to produce superfluorescent light from a light source with very low relative intensity noise. Such applications for a superfluorescent source include fiber optic gyros for high precision air and sea navigation and strain sensing for any of a number of structures such as ships, airplanes, bridges, highways, or in general any civilian or military structure.
The fluctuations in amplitude of conventional superfluorescent sources can cause relative intensity noise (RIN), which in turn degrades sensor performance. One approach for removing this type of noise is noise subtraction which requires a long fiber delay and has been shown to be insufficiently effective. See for example, Moeller, R. P. and Bums, W. K. 1.06-xcexcM all-fiber gyroscope with noise subtraction. Optics Letters 16(23), pages 1902-1904, 1991.
Other approaches to reduce noise in lasers in general include a xe2x80x9cpower stabilizerxe2x80x9d, which is a commercially used feedback approach that taps out part of the beam and controls an electronically variable attenuator, through a servo controller, in the output beam. See Miller, P. and Hoyt, C. Turning Down Laser Noise with Power Stabilizers. Photonics Spectra, June 1986, pages 129-134. This approach has been used to reduce RIN at low frequencies in a diode pumped yttrium-aluminum-garnet (YAG) laser, where the variable attenuator was a bulk-optic, electro-optic modulator. See Dagenais, D. M. et al., Low-frequency Intensity Noise Reduction for Fiber-Optic Sensor Applications. Proc. OFS ""92, #P26, pages 177-1180, Monterey, 1992.
For waveguide sources where the output of the source is already in a fiber an appropriate electro-optic modulator can be used such as an integrated optical (IO) Mach Zehnder interferometer. This approach, however, is disadvantageous because of the excess loss of the modulator (typically xe2x88x923 dB) and an additional loss (about xe2x88x923 dB) to operate in the linear region of the modulator transfer curve (i.e., at quadrature). Another difficulty is polarization in that the output of the fiber superfluorescent source is unpolarized, while the IO interferometer has polarization dependent transfer and drive voltage characteristics.
It would thus be desirable to provide new methods and devices that yielda high power, broadband, optical light source with very low RIN. It would be particularly desirable to provide such a device and method that would yield such an optical source without a loss of intensity, and that can maintain a polarized state in comparison to prior art devices. Such devices preferably would be simple in construction and less costly than prior art devices and such methods would not require highly skilled users to utilize the device.
The present invention features a superfluorescent light producing device and a method for reducing relative intensity noise (RIN) in a superfluorescent light source that can provide a polarized optical output more particularly a broadband polarized optical output. A superfluorescent device/source according to the present invention is particularly advantageous for applications such as high precision navigation and low noise strain sensing. In the present invention, the term broadband shall be understood to mean an output, more particularly an optical output, extending over a range of wavelengths and frequencies. In particular, the term broadband can refer to a bandwidth of the range of wavelengths of least 20 nm.
A superfluorescent light producing device or source according to the present invention includes a seed source, a modulator and a polarization maintaining amplifier. The seed source includes a light source capable of producing a light or optical output at a preselected wavelength/frequency and a first doped optical fiber that is doped with a preselected gain material and optically coupled to the light source. The first doped optical fiber, responsive to the light output from the light source, provides a broadband optical output over a range of pre-selected wavelengths and frequencies, this optical output comprises and is referred to hereinafter as the seed source optical input.
In more particular embodiments the seed source light source comprises a laser pump diode, the pre-selected gain material or dopant is one or more materials that provide an amplified spontaneous emission (ASE) or optical output at desired wavelengths and frequencies and the first doped optical fiber is optically coupled to the laser pump diode. In a more specific embodiment, the dopant is erbium, however, it is within the scope of the present invention for the dopant to include any rare-earth material, including but not limited to holmium, neodymium, praseodymium, and ytterbium.
In an exemplary embodiment, the laser pump diode is configured so as to provide a light output at about 980 nm and the first doped optical fiber is an erbium doped fiber. The 980 nm light output from the light source is inputted to the erbium doped fiber so as to cause an ASE from the erbium doped fiber. The ASE optical output from the erbium doped fiber provides an optical output at about 1550 nm that is counter propagated towards the laser pump diode in the erbium doped fiber.
The input of the modulator is operably and optically coupled to the seed source so as to receive the seed source optical input. The seed source optical input is processed within and selectively passed through the modulator so that a polarized optical output is provided therefrom. In specific embodiments, the modulator is an electro-optic modulator that also can modulate the polarized optical output.
The output of the modulator is operably and optically coupled to the polarization maintaining (PM) amplifier so the polarized optical output from the modulator is propagated to the PM amplifier. The PM amplifier, including the components making up such an amplifier, is configured and arranged so as to in effect amplify the polarized optical output from the modulator and to provide an amplified polarized optical output that can be used for a given application. This amplified polarized optical output comprises the optical output of the superfluorescent light producing device/source of the present invention.
In an exemplary embodiment, the PM amplifier includes a beamsplitter, an amplification light source being capable of producing light at a preselected wavelength, a second doped optical fiber that is doped with a preselected gain material and a retro-reflecting orthogonal polarization converter that are operably and optically coupled to each other. As indicated above, these components are configured and arranged so as to in effect amplify the modulator""s polarized optical output and to provide the amplified polarized optical output.
In this exemplary embodiment, the beamsplitter is configured so that the polarized light from the modulator passes therethough and onto the second doped optical fiber. The amplification light source is disposed between the beamsplitter and the second doped optical fiber and injects light of a predetermined wavelength and frequency into the second doped optical fiber. The injected light from the amplification light source excites the rare-earth dopants in the second doped optical fiber such that an amplified broadband optical output is outputted by the second doped optical fiber.
The amplified broadband optical output from the second doped optical fiber is propagated to the retro-reflecting orthogonal polarization converter. The retro-reflecting orthogonal polarization converter reflects the amplified broadband optical output from the second doped optical fiber in an opposite direction and so the reflected amplified optical output is in an orthogonal polarization state (i.e., orthogonal with respect to the polarization state of the polarized optical output incident on the polarization converter. As the reflected amplified optical output passes back through the second doped optical fiber in the opposite direction, the reflected amplified output is further amplified. It should be recognized that in an exemplary embodiment the reflected amplified optical output has a different orthogonal polarization than that characterizing the polarized optical output from the modulator.
The retro-reflecting orthogonal polarization converter is optically coupled to the beam splitter such that this reflected amplified optical output in the orthogonal polarization state is inputted to the beamsplitter. The beamsplitter also is configured so that the reflected amplified optical output having the orthogonal polarization state is directed into another optical pathway. The other optical pathway is arranged so as to be at an angle (for example 90xc2x0) with respect to the optical pathway that inputs the polarized optical output from the modulator to the beamsplitter. The reflected amplified polarized optical output being outputted by the beamsplitter comprises the polarized light or polarized, amplified optical output of the superfluorescent light producing device/source of the present invention.
In a particular embodiment, the amplification light source is a laser pump diode, the second doped optical fiber is a doubleclad rare-earth doped optical fiber and the retro-reflector is a Faraday mirror, more particularly a Faraday rotator mirror. In a specific embodiment, the rare-earth dopants are erbium and yetterbium, however, it is within the scope of the present invention for other rare-earth materials, alone or in combination, to be utilized to dope the optical fiber.
In the foregoing, it is provided that components making up the seed source and PM amplifier are optically coupled as well as that the seed source, the modulator and PM amplifier are optically coupled to each other. It is within the scope of the present invention for any mechanism, device or material known in the art for accomplishing such optical coupling or interconnecting to be used or adapted for use with the superfluorescent light producing device/source of the present invention. In an exemplary embodiment, optical coupling is accomplished using an optical fiber such as a polarization maintaining optical fiber.
The superfluorescent light producing device/source according to the present invention further includes a control mechanism that automatically adjusts or modulates the polarized optical output from the modulator thereby modulating the amplified polarized optical output from the superfluorescent source to a desired value. More particularly, a portion of the reflected amplified polarized optical output from the beamsplitter is tapped-off and used to control the modulator by means of a feedback loop, more particularly to control the transmission of the modulator by means of the feedback loop.
In an exemplary embodiment, the feedback loop includes a detector and a noise feedback amplifier. The removed portion is converted into an electrical signal by the detector, which electrical signal is proportional to the intensity of the reflected amplified polarized optical output. This electrical signal is fedback to the modulator via the noise feedback amplifier thereby controlling the transmissivity of the modulator. In this way, amplitude fluctuations or relative intensity noise (RIN) in the reflected amplified polarized optical output that can be attributable to any one of a number of causes can be minimized automatically by the superfluorescent light producing device/source of the present invention.
Other aspects and embodiments of the invention are discussed below.